The goal of this project is to identify and characterize the genetic basis for certain of the primary immunodeficiency disorders, which are associated with an increased incidence of cancer. While these disorders are rare, their study has been extremely instructive in defining previously unsuspected elements of importance in human immune function. The Immunophysiology Section conducts a clinical translational research program. We have identified the DNA repair enzyme Msh5 as being defective in patients with Common Variable Immune Deficiency, one of the most common primary immune deficiency diseases associated with an increased incidence of cancer. We have shown that approximately 11% of patients with another primary immune deficiency disease, the Wiskott-Aldrich syndrome, have spontaneous genetic reversions in their mutated genes.